Hitting the books.
Studying hard.
Don't let the cat out of the bag.
It means don't tell the secret or don't ruin the surprise.
It costs an arm and a leg.
It's expensive.
You can say that again!
I agree!
Don't "throw in the towel!"
Don't give up!
It's raining cats and dogs.
It's raining hard.
Hold your horses.
Wait.
You can't teach an old dog new tricks.
It's hard for old people to learn something new.
He got up on the wrong side of the bed.
He's grouchy.
Break a leg.
Good luck!
Let's pig out!
Eat a lot!
You're the "teacher's pet."
The teacher's favorite student.
Give it a shot.
Try it.
The world is your oyster.
You can achieve anything.
In hot water.
In trouble.
She is "catching some Z's."
She is sleeping.
Multiplication is a "piece of cake."
It is not hard, it's easy!
He is feeling "under the weather."
He feels sick.
I'm all ears.
I'm listening.
You're pulling my leg.
You're joking.
Hit the sack.
Go to sleep.
It slipped my mind.
I forgot.